#include <iostream>
using namespace std;
int getDay(int day,int inday){
	int r;
	if(inday>=day){
		r=day+1;
	}else{
		r=day;
	}
	return r;
}

int getinDay(int day,int inday){
	int r;
	if(inday<day){
		r=inday+1;
	}else{
		r=1;
	}
	return r;
}
long long allMoney(int end){
	long long total=0;
	int day=1,inday=0,money;
	for(;end!=0;end--){
		int nextDay=getDay(day,inday);
		int nextinday=getinDay(day,inday);
		money=nextDay;
		day=nextDay;
		inday=nextinday;
		total+=money;
	}
	return total;
}

int main(){
	int end,summoney;
	cin>>end;
	summoney=allMoney(end);
	cout<<summoney;
	return 0;
}
